All lead-acid batteries will naturally self-discharge which can result in loss of capacity from sulfation. The rate of internal self-discharge is most influenced by the temperature of the battery's electrolyte and the chemistry of the plates. Heat speeds up this self-discharge, so do not store batteries in a hot garage or warehouse. This internal self-discharge is often mistaken for concrete floor causing the battery to drain. Some experts believe that storing batteries on a colder surface might actually slow down the self-discharge (leakage) rate because the floor acts as a heat sink and cools the battery. (Please see Section 13 for more information on storing batteries and Section 1 for more information on sulfation.

In the early 1900s, battery cases were made of porous materials such as tar-lined wood boxes, so storing batteries on concrete floor would accelerate their natural self-discharge due to external leakage. Modern battery cases are made of polypropylene or hard rubber and are better sealed and insulated. External leakage-causing discharge is no longer a problem storing batteries on concrete floors, provided the top of the battery is clean and free from wet or dried electrolyte.

Large differences in temperature could cause electrolyte stratification within very large batteries (>250 AH) which could accelerate its internal "leakage" or self-discharge if the battery is sitting on an extremely cold concrete, stone or steel floor in a warm room, boat or submarine. Stirrers or bubblers are often used on these types of large batteries to keep the electrolyte from stratifying. Undercharging will also cause electrolyte stratification, which can also result in loss of capacity from sulfation.

There are a number of factors affecting a vehicle charging system's ability to recharge a battery, such as the electrical load, how much power from the alternator is diverted to the battery, how long the power is available, and the temperature. Generally, idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery. This will leave your battery undercharged which causes a gradual accumulation of sulfation. When a dead battery needs to be recharged, it is best to use an external battery charger because you could overheat and damage your vehicle's charging system and you will save a lot of fuel and wear and tear on your engine. Please see Section 5 for more information on vehicle charging systems and Section 9 on charging.

If jump starting is required to start an engine, the battery should be fully charged by an external charger and then tested for latent damage. Assuming that a car battery has a 50 amp hour capacity and the vehicle's charging system is capable of recharging it at 50 amps at highway speeds, it would take approximately 120 minutes to fully recharge a healthy battery. If the battery is frozen, do not attempt to jump start the engine or recharge the battery. Install another fully charged battery until the frozen battery can be thawed out, fully recharged, and tested or tow the vehicle to a heated garage. Vehicle charging systems are not designed to recharge fully discharged batteries and by doing so, you may damage the alternator.

Lead-acid batteries can provide source of fuel for external or internal explosions. Charging a wet lead-acid battery produces hydrogen and oxygen gasses as electrolysis of the water in the electrolyte occurs. That is why recharging needs to occur in well ventilated areas with the vent caps on the battery. While spark retarding vent caps help prevent external battery explosions, a spark can ignite the hydrogen gas when jumping, connecting or disconnecting a battery charger, or battery cables. From the U.S. Department of Energy, DOE-HDBK-1084-95, "Precautions must be routinely practiced to prevent explosions from ignition of the flammable gas mixture of hydrogen and oxygen formed during overcharge of lead-acid cells. The maximum rate of formation is 0.42 Liters of hydrogen and 0.21 Liters of oxygen per ampere-hour overcharge at standard temperature and pressure. The gas mixture is explosive when hydrogen in air exceeds 4% by volume." [The ignition point of Oxygen occurs above 23.5% concentration.] Less common internal explosions usually occur while starting the engine or if the battery is rapidly heated from a short circuit or fire. Normally, a battery explosion will damage the battery and splatter electrolyte all over the engine compartment or battery box. Please see Dr. Charles E. Roberts Jr.'s article "Investigating Automotive Battery Explosions" for more information on car battery explosions.

The most probable cause of internal wet battery explosions are from a combination of electrolyte levels below the plates, and a low resistance bridge is formed between or across the top of the plates, and a build up of hydrogen gas in the cell. The low resistive bridge or "treeing" occurs between the positive and negative plates. When current flows in the battery, residual gas is ignited in one or more of the cells. A second possible cause is a manufacturing defect in the weld of one of the plate connecting straps causing a spark and hydrogen ignition. Another source of internal battery explosions is caused from a direct electrical short across the battery's terminals. The battery rapidly over heats form the high current and explodes. A less common form of internal battery explosion occurs when the battery is in a fire or subjected to high temperatures while charging causing thermal runway in VRLA (AGM or Gel Cell) batteries. The largest number of internal battery explosions occur in hot climates. This could be attributed to increased consumption of water in sealed wet "Maintenance Free" and low maintenance starting batteries. Most internal wet battery explosions could have probably been prevented if the plates were always covered with electrolyte. Please see Section 3 for more information on preventive maintenance.

While not fatal, each year battery explosions cause tens of thousands of eye and burn injuries from the electrolyte (battery acid) globally. According to PREVENT BLINDNESS AMERICA, nearly 6,000 motorists suffered serious eye injuries annually from working around car batteries. Should a battery explosion occur and battery electrolyte (battery acid) gets in the eyes, flush with any drinkable liquid immediately because SECONDS count, continue flushing with water for at least 15 minutes, and seek immediate medical attention. In addition, neutralize residual battery acid, by thoroughly washing the engine compartment and the back of the hood with a solution of one-pound baking soda (bicarbonate of soda) to one gallon of warm water and rinse thoroughly with water. Proper periodic preventive maintenance (please see Section 3.), working on batteries in well-ventilated areas, or using non-sealed wet "Maintenance Free" or sealed AGM (Ca/Ca) or Gel Cell (Ca/Ca) type batteries can significantly reduce the possibility of battery explosions. All lead-acid batteries require periodic maintenance.

Depending on the type of battery and temperature, batteries have a natural internal self-discharge (electrochemical "leakage") at a rate of 1% to 60% per month. Over time the battery will become sulfated and fully discharged which will make it more susceptible to freezing. Higher temperatures will significantly accelerate the self-discharge process. A battery stored at 95° F (35° C) will self-discharge twice as fast than one stored at 75° F (23.9° C). Leaving a battery connected in a vehicle can also increase the discharge of battery due to the additional parasitic (ignition key-off) load. The addition discharge from a parasitic load can be eliminated by disconnecting the negative cable from the battery terminal, but will not affect the internal self-discharge. If the battery is stored for more than three months or in warm temperatures, periodic or "float" changing will prevent the self-discharge from permanently damaging the battery. (Please see Section 15 and Section 16 for more information on parking times and sulfation.)

All lead-acid batteries require some preventive maintenance, some types more than others. The term "Maintenance Free" generally refers to a wet, sealed lead-acid car and deep cycle batteries with calcium added to the positive and negative plates. (Please see Section 7.1.3 for more information on these types of batteries.) In hot climates, the water is lost due to evaporation caused by high underhood temperatures and normal charging. Water can also be lost due to excessive charging voltage or charging currents. Using non-sealed wet Low Maintenance (Sb/Ca) batteries (with filler caps) is encouraged in hot climates so distilled water can be added when this occurs. (Please see Section 3. for other preventive maintenance procedures that should be performed on lead-acid batteries.)

A battery, aside from providing power to start the engine and power when the electrical load exceeds the charging system output, acts as a electrolytic capacitor to stabilize or filter to the pulsating DC voltage produced by the alternator. Disconnecting a battery while the engine is running coulddestroy the sensitive electronic components connected to the electrical system such as the emission computer, radio, audio system, cell phone, alarm system, etc., or the charging system because the peak voltage can rise to 40 volts or more. Prior to 1980, removing a battery terminal was an accepted practice to test charging systems of that era. That is not the case today. Static electricity and spikes from connecting and disconnecting batteries or test equipment could also damage sensitive electronic components.

14.7. Will conditioners, aspirins or additives will revive sulfated batteries?

Most battery experts agree that there is no evidence that conditioners, additives or aspirins provide any long-term benefits for heavily sulfated batteries. Short term gains, if any, are achieved by increasing the acidity (Specific Gravity) of the battery, which could increase the Amp Hour capacity, but also increase the water consumption and positive grid corrosion; thus, decreasing the overall service life of the battery. If a battery will not take a charge, then it is best to replace it with a healthy battery. This controversy between the additive manufacturers, battery manufacturers, and independent electrochemists has been going on for over 60 years as demonstrated in this AD-X2 Battery Additive, From a Trickle to a Torrent article from the National Institute of Standards and Technology (NIST) Museum.

14.8. On really cold days, should headlights be turned on to "warm up" the battery up before starting the engine?

Turning on the headlights will increase the current flow in a battery, it also consumes valuable capacity that could be used to start the cold engine; therefore, this is not recommended. For cold temperatures, externally powered temperature compensated battery "float" chargers, warmers or blankets, and engine block heaters are highly recommended if the vehicle can not be parked in a heated garage. AGM (Ca/Ca) and Ni-Cad batteries will perform better than wet lead-acid batteries in extremely cold temperatures.

Car batteries last an average of two thirds as long in hot climates as cold ones due to increased grid corrosion and water loss. Heat kills batteries, especially sealed wet "Maintenance Free" (Ca/Ca) batteries, and cold reduces the battery's starting performance. (Please see Section 11.1 for more information on increasing battery life.)

The cigarette lighter charging cable's advertising states "charges weak batteries in minutes." Charging cable products will certainly increase the charge in your car battery if charging current and proper voltage is applied long enough and your battery is in good condition. Cigarette lighters are normally fused at 10 amps, so to be safe they probably limit current to flow less than the fuse size. Given the diameter of the wire used in the cable, the amount might be even less.

Charging cables will only work by applying higher voltage from the vehicle with the charged battery to "recharge" dead battery. In order to recharge a battery, the charging voltage needs to be approximately two volts greater than the battery voltage to overcome the internal resistance of the battery. Let's assume it is a hot day and that you need just of 3% of the battery's capacity to start the engine from a 40 amp hour battery. This means you will need at least 10 minutes to flow power from the charged battery with the engine idling fast enough to produce 7.5 amps to the discharged battery. Now let's assume that it is below freezing and you have left your lights on. Assuming the dead battery is not frozen, you will need at least 50% capacity or 20 amp hours to start the vehicle. This will take over two hours to partially recharge the dead battery. Using jumper cables with the engine running at higher idle speed will partially charge a dead battery much faster because more current can be applied to the discharged battery. Please see Section 6 for jump starting, but be sure the battery is not frozen or the case is not cracked.

A starter motor will only use a fixed amount of current from the battery, based on the resistance of the motor. A higher Cold Cranking (CCA) performance or greater Reserve Capacity (RC) or Amp Hour (AH) capacity battery supplies only what is required. This will not damage your vehicle; however, using batteries with higher or lower voltage or physically too high could potentially cause harm. For example, using a 24-volt battery or 6-volt battery in a 12-volt vehicle electrical system.

Lead-acid batteries do not have the "memory effect" mistakenly identified with first generation Ni-Cad batteries. Months of continuous undercharging will lower the capacity of the battery over time due to the accumulation of permanent lead-sulfate or "sulfation". Deep discharges below twenty percent State-of-Charge (approximately 12.0 volts) can damage car batteries and will significantly shorten their service lives.

A bad or weak starting battery causes additional stress on a charging system, starter motor or starter solenoid. It can cause premature failures due to compensating for the voltage or current. If you replace a battery, alternator, voltage regulator or starter, you should test the other components for damage and repair or replace as required.

If a battery is fully discharged and continues to have a load, for example leaving the headlights on, it is possible for one or more cells to reverse polarity. This is referred to as "cell reversal". To change polarity, fully discharge the battery and recharge it with an external battery charger with the correct polarity.

Use only distilled, deionized or demineralized water to replace the lost water in batteries. This is because using tap or reverse osmosis water from residential systems can produce calcium or magnesium sulfate crystals that can fill the pores and coat the plates. Wet batteries will have a longer service life if you do not use tap water. If the plates are not covered and distilled water is not available, use rain or tap water to prevent sulfation and internal battery explosions.

14.16. Why Are Vehicles Negatively Grounded?

The best explanation to this question comes from a 1978 Rolls-Royce Enthusiasts' Club service manual.

"...it has been found that cars wired positive earth [ground] tend to suffer from chassis and body corrosion more readily than those wired negative earth. The reason is perfectly simple, since metallic corrosion is an electrolytic process where the anode or positive electrode corrodes sacrificially to the cathode. The phenomenon is made use of in the "Cathodic Protection" of steel-hulled ships and underground pipelines where a less 'noble' or more electro-negative metal such as magnesium or aluminum is allowed to corrode sacrificially to the steel thus inhibiting its corrosion."...